A variety of ring laser systems have been developed in the prior art. In a ring laser, a plurality of reflecting surfaces are provided for guiding the laser light about a closed loop. The laser light in a ring laser takes the form of a travelling wave. This situation can be compared with the more common linear laser cavity where the light energy bounces back and forth between two mirrors defining the cavity. The light in a linear cavity forms a standing wave.
One primary advantage of a ring laser is that travelling wave operation can yield single frequency performance. Single frequency performance is desirable when a non-linear crystal is placed in the laser cavity for generating second harmonic radiation. Attempts to generate second harmonic radiation using a linear cavity have typically resulted in significant amplitude fluctuations due to longitudinal mode coupling. (See, "Large-amplitude Fluctuations Due to Longitudinal Mode Coupling in Diode-pumped Intracavity-doubled Nd:YAG, " T. Baer, J. Opt. Soc., Vol. 3, No. 9, 1986) Ring lasers, which generate single frequency output avoid mode beating, allowing for a more stable frequency doubled output.
As will be discussed below, single frequency performance is possible when a means is provided to insure that the light travels in only one direction in the ring. If such a means is not provided, two counterpropagating beams will travel in the ring adversely affecting performance.
One of the principle drawbacks to ring lasers is that they can be difficult to align and operate. As noted above, a ring laser will have a number of reflecting faces, each of which must be properly aligned for effective operation. In addition, the increased number of optical components necessary to define the ring will increase losses.
One approach for minimizing the problems associated with prior art ring lasers is to use a monolithic ring structure. In a monolithic structure, a single block of lasant material is formed with integral reflecting surfaces. As can be appreciated, no adjustments are necessary in a monolithic structure. Also, losses are minimized.
One example of a monolithic ring laser is disclosed in U.S. Pat. No. 4,578,793, issued Mar. 25, 1986 to Kane et al., the disclosure of which is incorporated herein by reference. The monolithic structure disclosed in the patent has three facets that totally internally reflect the laser beam. A fourth face acts as both the input and output coupler. The light travels within the monolithic structure along a non-planar ring path.
The device described in the latter patent also includes a means to insure that the light travels unidirectionally. This means includes providing a magnetic field about the lasant material to generate a nonreciprocal polarization rotation based on the Faraday effect. By nonreciprocal polarization rotation it is meant that the direction of rotation of the polarization induced by the magnetic field is independent of the direction in which the light is travelling.
In order to obtain unidirectional operation, the ring must also include a means for generating a reciprocal polarization rotation. By reciprocal polarization rotation it is meant that the direction of rotation will be dependant upon the direction which the light is travelling. More specifically, polarization will be rotated in one direction when the light is travelling one direction in the ring and when the light is travelling in the other direction, the rotation will be exactly reversed.
Reciprocal polarization rotation can be achieved in a non-monolithic ring by using a conventional optically active rotator, a birefringent element or a non-planar ring geometry. In the Kane device, the non-planar geometry of the reflecting facets are arranged to create the necessary reciprocal polarization rotation effect. By providing both reciprocal polarization rotation and nonreciprocal polarization rotation, the light energy travelling in opposite directions in the ring will have different polarization orientations.
The third and final element necessary to obtain unidirectional operation is a means for providing polarization discrimination. In a standard ring, polarization discrimination can be created with an optical element inclined at Brewster's angle. In the monolithic Kane device, the light incident on the output coupler face is inclined at an oblique angle and the output coupler face is optically coated to provide polarization discrimination. By providing polarization discrimination, light having one polarization orientation (and direction of travel about the ring) will have higher losses than light in the counterpropagating beam and therefore lasing will be suppressed. In this manner, unidirectional operation is achieved which is necessary for single frequency operation. As will be discussed below, the ring laser of the subject invention preferably includes an equivalent means for inducing unidirectional operation.
While devices designed in accordance with the Kane patent are useful, they have certain drawbacks. For example, efficient intracavity frequency doubling cannot presently be achieved in a monolithic structure. Generation of the second harmonic is typically limited to the use of frequency doublers located outside of the resonant cavity where power is lower and doubling efficiency is greatly reduced.
Very recently, there has been some work performed in developing ring laser geometries which would be easier to align and allow intracavity frequency doubling and Q-switching. This work is described in "Single Frequency Q-Switched Operation of a Laser Diode-Pumped, Nd:YAG Ring Laser," Clarkson and Hanna, Optics Communications, Vol. 73, No. 6, Nov. 15, 1989. The laser described therein has a minimum of elements so that alignment is relatively straightforward.
FIG. 1 illustrates the ring laser 10 described in the Clarkson paper. The laser consists of a Nd:YAG laser rod 12 having an end face 14 coated for high reflectivity at the laser wavelength and high transmission at the pump wavelength. The rod is pumped by a beam from a diode laser (not shown). The other end of the cavity is defined by a concave output coupler 16. A rhombic prism 20 is located between the rod and the output coupler.
In the Clarkson device, the geometry of the prism 20 serves to define the geometry of the ring. More specifically, the angles of the prism are chosen so that the laser beam is deflected by refraction into a ring path. The faces of the prism are chosen to be at Brewster's angle to minimize losses and act as a polarizer. In the illustrated embodiment, the prism is formed from lithium niobate and can be used to Q-switch the laser by application of the appropriate voltage which causes the prism to act as a Pockels cell.
Unidirectional and single frequency operation is achieved in a manner analogous to that described above. More specifically, nonreciprocal polarization rotation is induced through the presence of a magnetic field generated by permanent magnets 22. Reciprocal polarization rotation can be induced from the birefringence of the lithium niobate prism or by placing a slight wedge on the face of the rod creating a non-planar ring geometry as described in the Kane patent. The polarization rotations combined with the losses induced from the Brewster faces of the prism will create a differential loss between the counterpropagating beams and will thus induce unidirectional operation.
As can be appreciated, the device shown in FIG. 1 has less reflecting elements than in prior art ring laser structures. The device also allows for the insertion of intracavity elements not possible with the monolithic device disclosed by Kane. However, the device in FIG. 1 does have drawbacks. For example, the ring geometry is achieved using a separate additional optical element (i.e. the prism 20.) It would be desirable to create a ring geometry with even fewer components.
Another drawback to the geometry of FIG. 1 is that the beam must pass through the prism media twice, thereby increasing the loss in the system.
A further drawback of the Clarkson approach arises if this geometry were to be used in a frequency doubling scheme wherein the rhombic prism was formed from a non-linear optical material. For example, while the Brewster angle faces of the prism serve to minimize optical losses by reflection for uncoated optical elements, they will impart asymmetries to the beam within the prism which in turn adversely affect the ability to focus the beam. The ability to tightly focus the beam is required to maximize doubling efficiency.
Other drawbacks would be encountered depending upon the non-linear material selected. For example, a Type II phasematching material such as KTP exhibits double refraction wherein the two polarization states of the resonated fundamental beam will be refracted at different angles. The angular separation between the two refracted beams typically increases as the angle of incidence is increased. Thus, the requirement of a Brewster's angle of incidence would create severe double refraction leading to high intracavity losses. Moreover, the double refraction problem would be compounded in the Clarkson scheme since the beam must pass through the crystal a second time on the return path.
The Clarkson geometry is also undesirable for uniaxial, non-critical Type I phasematching materials such as lithium niobate. In these types of crystals, the phasematching conditions will be satisfied for both beam paths through the prism. This will result in the second harmonic wave being generated in both beam direction passes through the prism, thereby reducing the efficiency of the second harmonic output. Accordingly, it would be desirable to provide a new ring laser design that overcame the difficulties described above with respect to the approach shown in FIG. 1.
Therefore it is an object of the subject invention to provide a new ring laser design which can have fewer optical elements.
It is another object of the subject invention to provide a new ring laser design which allows the beam to be tightly focused within the non-linear crystal.
It is a further object of the subject invention to provide a new ring laser design that has a minimum of refractive interfaces and does not require passing the beam through the refractive optical element more than once.
It is still another object of the subject invention to provide a ring laser design which can be pumped by a laser diode.
It is still a further object of the subject invention to provide a ring laser design having an intracavity non-linear optical material and which generates stable second harmonic output.